The present invention relates to a membrane process and system for the purification of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen for fuel cells. The purification process selectively reomves CO2 from the reformed product thereby enriching the reformed product in H2 and increasing the H2/CO2 ratio. This invention also relates to a polymer composition suitable for forming a membrane that is useful for separating CO2 from a CO2-containing gas stream in the purification process. The present invention is particularly useful when the process is carried-out on-board a vehicle using a fuel cell for transportation.
Reforming of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen is generally proceeded with the formation of the synthesis gas of CO and H2 first. For example, steam reforming of methanol with a NiO/Al2O3 catalyst at 300-400xc2x0 C. (T. B. Su and M. H. Rei, J. Chin. Chem. Soc. (Taipei), 38, 535 (1991)) gives the synthesis gas:
CH3OHxe2x86x92CO+2H2xe2x80x83xe2x80x83(1)
Steam reforming of CH4 with a nickel-based catalyst at 800xc2x0 C. is:
CH4+H2Oxe2x86x92CO+3H2xe2x80x83xe2x80x83(2)
Partial oxidation of CH4 is:
xe2x80x83CH4+xc2xdO2xe2x86x92CO+2H2xe2x80x83xe2x80x83(3)
Similarly, partial oxidation of other hydrocarbons, e.g., gasoline and diesel, produces the synthesis gas:
CnH2n+2+{fraction (n/2)}O2xe2x86x92n CO+(n+1)H2xe2x80x83xe2x80x83(4)
where n is an integer. In the partial oxidation, the synthesis gas produced does not contain N2 when O2 is used. If air is used instead of O2, the synthesis gas produced contains N2.
The synthesis gas is then sent conventionally to two-stage water gas shifters, in which CO is converted to CO2 via the water gas shift reaction:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(5)
Typically, the first-stage shifter operates at higher temperature than the second- stage shift, e.g., 373xc2x0 C. for the first stage and 225xc2x0 C. for the second stage. For the water gas shift reaction, CuO/ZnO/Al2O3 catalysts can be used. The product gas from steam reforming of methanol under the optimum conditions at 1 atm and 227xc2x0 C. with a water rich feed (water/methanol =1.5) contains approximately 66% H2, 21% CO2, 1% CO, and 12%,H2O (J. C. Amphlett, M. J. Evans, R. A. Jones, R. F. Mann, and R. D. Weir, Can. J. Chem. Eng., 59, 720 (1981)).
In some reforming cases, such as the steam reforming of methanol with a CuO/ZnO/Al2O3 catalyst, methanol is converted directly and predominantly to CO2 and H2: 
This reaction operates at temperatures lower than 260xc2x0 C. with methanol conversion as high as 90%; however, trace CO appears at temperatures above 300xc2x0 C. and high methanol conversions of about 90% (C. J. Jiang, D. L. Trimm, and M. S. Wainwright, Appl. Catal. A, 93, 245 (1993)).
Japanese Patents 04,321,502 and 04,325,402 claim processes employing H2-selective membranes, which selectively pass H2 and reject other gases, for hydrogen manufacture for fuel cells. However, these processes suffer from a low pressure for the H2 product gas which is much lower than the pressure for the feed gas. Thus, a compressor is needed to compress the product gas to the pressure of the feed gas. In addition, these processes also usually have other shortcomings, such as low H2 recovery, large membrane areas, and a high CO2 concentration in the product gas.
It is an object of the present invention to provide a CO2-selective membrane process that selectively passes CO2 over H2 and other gases and that is useful for the purification and/or water gas shift reaction of a reformed gas, generated from reforming of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen for fuel cells. This CO2-selective membrane process can be more advantageous than H2-selective membrane processes in terms of H2 product pressure (to avoid the need of a compressor for the product gas), H2 recovery, membrane area, and CO2 concentration. Another object of the present invention is to provide a novel polymer composition that is suitable in formation of a membrane useful for the CO2-selective membrane process. Membranes disclosed in U.S. Pat. No. 5,611,843 may also be used for the CO2 selective membrane process.
The present invention is a process and system to purify a fuel feedstream so that the feedstream is enriched in H2. In general, the process includes the steps of reforming the feedstream, and separating CO2 with a membrane that selectively removes CO2 from the feedstream. For most fuel feedstreams, a step of water gas shift reaction is also included in the process. The CO2 selectively permeable membrane may also be used to perform both steps of enhancing water gas shift reaction and separating CO2. The process further comprises the step of methanating the H2-enriched feedstream.
If the fuel feedstream is methanol, then a water gas shift reaction step is not necessary. In this embodiment, the CO2 selectively permeable membrane may be used to perform both steps of reforming and separating.
In a preferred embodiment, the process is carried-out on board a vehicle that uses a fuel cell for transportation.
Another embodiment of the present invention is directed toward a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition. The composition is suitable in formation of a membrane useful for separating CO2 from a CO2-containing gas, particularly from an on-board reformed gas for the CO2-selective membrane process.
The embodiments of the present invention will become apparent upon a reading of the brief description of the drawings and the detailed description of the invention which follow.